On a brisk Thursday night outside Finsbury Park Mosque in north London, Abdul Qassim and his friend Mohamed Salim were talking of war. They look like typically articulate and casually dressed young Asians. But they want to fight for the Taliban.

'I'd never have previously considered going off to fight. But this is serious. Very serious. And something has to be done,' said Qassim.

Qassim, 26, and Salim, 22, are members of al-Muhajiroun, followers of firebrand Islamist Sheikh Omar Bakri Mohamed. They had gathered at the mosque along with 50 other young Asian men to hear their leader speak.

Fiercely uncompromising in their interpretation of Islam, the members of al-Muhajiroun are dedicated to their faith. But there can be a price to pay. Last week the group announced the death of at least three of its British members in Afghanistan. It said they had joined the Taliban to defend Islam, but were killed by an American bomb.

Qassim and Salim walked into the mosque and began to wash before prayer, removing their socks and shoes at a sink. Salim looked approvingly at the bearded Bakri, dressed in white robes and seated by the microphone.

'I have pledged myself to what he says. It's a promise and one that I intend to keep. I'd do anything he'd ask me to,' he said. The duo listened as Bakri began to condemn, with characteristic flair, Britain's war against terrorism.

'The British Government has to be stopped,' said Bakri. 'Blair knows that he is wrong. And he will pay for it. We will remodel this country in an Islamic image.' Waiting until his followers stopped giggling at the vision of an Islamic state of Great Britain, Bakri continued: 'We will replace the Bible with the Koran.'

Salim leaned inwards, whispering of his intention to fly to Lahore within weeks. 'I have managed to save some money from my job in a shop and I'll use that to get over there.' He looked round at Qassim in encouragement. 'You're going to do it as well, aren't you?' he asked. 'I think we should both go and fight. It's our duty to do it.' He glanced up at Bakri, who was now vociferously condemning Britain to an internal intifada . 'Christians have to learn that they cannot do this to Islam. We will not allow our brothers to be colonialised. If they try it, Britain will turn into Bosnia.'

Al-Muhajiroun was founded in Jeddah in 1983 by the charismatic, Syrian-born Bakri. It promises to re-establish 'true' Islam throughout the world to the extent of wiping out other religious faiths. It is extremely anti-Semitic. Bakri, who was expelled from Saudi Arabia and has lived in London since 1986, calls for young Muslims to take up arms against the opponents of Islam.

The organisation has offices across the developed world - in Kuwait, France, South Africa, Lebanon, Bangladesh, Mauritius, Syria and Algeria - and regularly asks its members for donations to fund its work. It funnels its British supporters to conflicts around the world by providing them with guides and contacts, but volunteers almost always have to pay their own way. Many commentators have previously written off Bakri and his tiny band of followers as an unpleasant joke. Bakri was even the subject of a documentary by the humorist Jon Ronson. But after the deaths last week, few people now see al-Muhajiroun as funny. Suddenly it is a threat.

To Al-Muhajiroun, Afzal Munir, Aftab Manzoor and Yasir Khan are martyrs who died defending their Islamic brothers against an attack by the infidel. They are to be glorified.

To many Britons, including government politicians, they are traitors, willing to take up arms to fight the armed forces of the country they grew up in. They are to be feared.

But to their parents, they were idealistic but perhaps mistaken young men, who gave up a life of suburban normality to die in a foreign field. They are to be mourned.

For Chudry Manzoor, who last week buried his son in the village of Sakria just outside Islamabad, it is a tragedy. He had repeatedly warned him about the risks, forbidding him to fight several times over the past three years. 'I never wanted him to fight a holy war against anybody,' he said at the burial.

For many, the most shocking thing about the three young men is the suburban normality of their lives. Chudry Manzoor is a Luton grocer who has lived in Britain for 20 years and brought Aftab up to be a respectful family man. Aftab, 25, had a variety of part-time jobs, including one as a driver. The family home - like those of the other two dead men - is on a quiet residential backstreet, leafy and modestly prosperous.

Aftab divided his time between Pakistan, where he had a wife and young daughter, and Luton, where he had gone to Denbigh High School and took his GCSEs. He worked hard and sent much of his earnings to Pakistan for his young family. He had left Luton for Pakistan for the last time before the 11 September attacks. But three weeks ago he telephoned his father and spoke of his plans to join the Taliban.

Afzal Munir, 25, was also from Luton. He still lived with his father - a builder - and his mother, three sisters and 11-year-old twin brothers. The crowded family home was less than a mile from Aftab's house. He was known as a quiet, friendly young man. He too had gone to a local school, Challney Secondary. He was a regular mosque goer and attended Friday al-Muhajiroun meetings. But even three weeks ago - when he was still in Luton - many of his friends had known little of his intentions to head for Afghanistan.

Some did, however. Mohamed Abdullah, 22, said the bombing of Afghanistan had affected him deeply. 'He may have lived in Luton but he felt the pain of his Muslim brothers and he wanted to do something about it,' he said. When Munir left for Pakistan he did not even tell his wife where he was going.

The third man in the trio, Yasir Khan, was from the Sussex commuter town of Crawley. He may have lived across the other side of London from the Luton pair, but Khan inhabited the same, seemingly quiet, suburban world. He lived with his mother in a maisonette. He was also a regular worshipper at the local mosque and had recently helped out with some renovation work. He was a keen cricketer with the Eagles Cricket Club, whose players come from Asian backgrounds. Three years ago a picture was taken at a club presentation night. The only hint of Khan's religious conviction was a t-shirt bearing the slogan: 'The Final Revelation, The Final Message, The Final System, The Final Conquest: Islam.'

Like scores of Crawley residents, Khan worked at Gatwick Airport. He had spent the last five months as a driver and loader for LSG Sky Chefs but, ironically, lost his job due to the downturn following the New York attacks. He had been asked to change his work schedule and refused. He is believed to have left for Pakistan shortly afterwards. His family insist that he was only on a mission to carry out aid work.

No one should have been surprised that Luton and Crawley produced Taliban fighters. Last year a Crawley family went to Pakistan to search for 18-year-old Omar Kyam. Al-Muhajiroun had sent him to fight in Kashmir. A Briton jailed in Yemen on bombing charges hailed from Luton. A suicide bomber in Kashmir was from Birmingham.

Al-Muhajiroun are effective because they exploit a sense of Asian victimisation, a feeling that - despite their qualifications, jobs and families - Asians will never be seen as properly British. It is a situation the older generation have tolerated. But young Asians like Manzoor, Munir and Khan, are less willing to accept the status quo.

'There is a sense that Asians have to be twice as good to be accepted as an equal, whether in accounting or football,' said Professor Tariq Modood of the University of Bristol's Ethnicity Research Centre.

That gives al-Muhajiroun an opening. Islam is the answer, they say. The West will not accept you, therefore reject the West. The Muslim world is where you belong. Come home to Islam. Come home to Afghanistan.

Near the banks of the River Ravi workers, hawkers and beggars push and shove down Lahore's traffic-clogged streets. In the din it is difficult to pick out the call to prayer. But in the plush suburb of Garden City the wail of 'Allah-u-Akhbar' can be heard loud and clear. And the men in the al-Muhajiroun office are listening.

One of their leaders is Hassan Butt, a 21-year-old Luton-born former student. He helped the three on their move to Afghanistan. Butt has been in Pakistan for the past eight months. He has been involved in al-Muhajiroun for far longer.

Butt, who grew up in Manchester and has A levels in media studies, computing and English, has been busy recently. There has been a steady flow of donations, including a cheque for £6,500 from a British-based doctor, and new people to be processed and sent to war.

Some are from Britain. Abdul Momin, a 25-year-old from east London - where al-Muhajiroun is known to have recruited among university students - arrived a month ago. Now the former civil engineer is preparing himself for jihad. 'I did not like London because it is spiritually rotten,' he said. 'I want to live a proper Muslim life.'

Despite the claims of al-Muhajiroun to have recruited up to 600 Britons to fight for the Taliban, the true numbers are much smaller, perhaps no more than a few dozen. Spokesmen for several Islamic groups with offices near to al-Muhajiroun headquarters all said there has been no flood of British volunteers.

But the fact remains that a few are making it through. The Britons killed in Kabul two weeks ago were with a group of Islamic fighters diverted from the guerrilla war in Kashmir. It had been simple to join them and head into Afghanistan through the gateway of Peshawar. From there it's a short ride in a pickup and a few hours' mountain hiking.

The handful of British volunteers for the Taliban are unlikely to have any material impact on the war in Afghanistan. They are poorly trained, poorly equipped and few in number. A British deserter last week described the problems he had faced when he was taken to the front line with other overseas novices and told to shoot at Northern Alliance soldiers. The result had been a bloody shoot-out, with the Taliban losing.

But the real danger from the British volunteers lies in the impact they can have on race relations. That is why the Government has threatened prosecution against any found to have fought and rumours have circulated about reviving long-dormant treason laws.

The nightmare scenario would involve British soldiers clashing with British Muslim volunteers, which experts warn could spark a disastrous breakdown in race relations. 'The ripple effect upon social attitudes, community segregation and race gang fights would be enormous, especially as many white people can't tell who is a Muslim and who just has Asian or Middle Eastern appearance,' said Modood.

But the divisive impact has already begun. In Luton the vast majority of Muslim leaders were outright in their condemnation of al-Muhajiroun. The group's leader in the town, a man known only by the name Shahed, has been forced into hiding.

'We cannot allow this generation to fall off our radar and leave them in the hands of extremist lunatic groups like al-Muhajiroun. That would be to condemn them to certain death,' said Dr Ghayasuddin Siddiqui, leader of the Muslim Parliament of Great Britain.

Yet many young Muslims may not be listening to their elders any more. At the Finsbury mosque Mohamed Salim now hears only the words of al-Muhajiroun. 'It seems to have given me a place in life. I was never very religious before, until I met someone from al-Muhajiroun. Now I think of the good of Muslims everywhere in the world.'

• This article was amended on 23 December 2010 to remove an incorrect reference to Afzal Munir attending Luton University, now the University of Bedfordshire. It was further amended on 1 February 2011 to remove a reference to Munir and Barnfield College.